I get a lot of questions from folks who are confused about the actual functionality of EIGRP Variance. So I thought I'd share my comments about it with a larger audience. This is Part-1 of that topic.

Enjoy!

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It's a common misconception that the "variance" command changes the distance of an EIGRP route. It does not. One would use the "offset-list" command to do that (a CCNP-level command).

Variance is a bit like this...

Imagine that you knew of several different ways to get information to your friend. The quickest method you were aware of took 1-minute to get the information to that person (i.e. making a phone call). You also knew of several other methods that took longer (Driving down the street to your friend's house = 3 minutes, biking down the street = 6-minutes, and walking down the street = 15-minutes).

Normally, if you needed to send information to your friend, you'd only select (for yourself) the fastest method (1-minute of a phone call). But what if I told you, "I have multiple pieces of information I want you send to your friend, but you can only use the phone call for one of them. For the remaining pieces of info, I want you to get them there using one of the other methods you know about."

The next question you might ask me would be, "is there some upper limit, some maximum time I'm allowed to get my information there? Or can I use any-and-all methods I know about?"

My reply is, "take your best method and multiply that time (in minutes) by seven (7). Any other method that you know about that is less than the product of that multiplication, you can use".

So you take your phone call (1-minute) and multiply it by seven and you get 7-minutes (your product). Looking at your remaining methods of information transfer to your friend you now know that you can also use the 3-minute drive, and the 6-minute bike. But you CAN'T use the 15-minute walk. You'd have to increase your "variance" to 16 to allow use of that method.

By using that variance of seven (7) have you changed how LONG it takes you to drive and bike to your friend's house? No. In your list of methods they still show up as 3-minutes and 6-minutes. But now you've been given PERMISSION to use those methods to get data (information) to your friend whereas before, you were limited to only using the BEST method (the phone call).

That's how variance works. It allows you to place lesser/alternative paths to a given destination in the IP Routing Table and use them for packet forwarding without actually chaning the distance of those alternative paths.

One other very important thing to note is that if an alternative path was determined by EIGRP to be a possible routing loop (i.e. did not meet the Feasibility Condition), you will never get EIGRP to use that route, no matter what crazy big number you put into the "variance" command.